Grade Level: 8th - 10th; Type: Chemistry
The goal of this experiment is to learn about atomic mass, atomic number, relative atomic mass and how different isotopes of an element affect the relative atomic mass.
- What is atomic mass? How does it differ from relative atomic mass?
- How can atomic mass be calculated?
- How does atomic mass differ from atomic weight?
- Is it possible for an element to have different atomic masses? Why?
- What percent of uranium is radioactive?
Students often confuse atomic number, atomic mass, atomic weight and relative atomic mass. The following definitions may be helpful:
Atomic number is the number of protons in an element. This number is always the same for every atom of a particular element; it is a fundamental property of the element.
Atomic mass is the total mass of protons, electrons and neutrons in an atom.Unlike atomic number, atomic mass is not a fundamental property of an element; rather, it is a fundamental property of a particular isotope.
An isotope is a particular form of an element. While all isotopes have the same number of protons, the number of neutrons can vary. For example, hydrogen has three isotopes: protium (with 0 neutrons), deuterium (with 1 neutron) and tritium (with 3 neutrons).
Atomic weight is the ratio of the average mass of atoms of a particular element to 1/12 the mass of an atom of carbon-12. While different samples of a particular element may have slightly different atomic weights, this number is sufficiently important that it appears on the periodic chart.
Relative atomic mass is a synonym for atomic weight. It represents the average of the mass of all isotopes weighted by the abundance of each isotope.
This “paper-and-pen” experiment does not require setting up any special apparatus. All that is needed is a periodic table.
Relative Atomic Mass of Silver
- The formula for relative atomic mass is the relative abundance over 100 divided by the isotope number. This can be expressed as:
(A/100 • a) + (B/100 • b) + (C/100 • c)…. = relative atomic mass
A, B, C stand for different relative abundances of isotope numbers a, b and c.
Relative Atomic Mass of Uranium
- Uranium has three isotopes: U 234, U235 and U238. The relative abundance of these is 0.01%, 0.71% and 99.27 % respectively. Use this information to calculate the relative atomic mass of uranium.
Relative Atomic Mass of Barium
- There are seven different isotopes of barium. Doing on-line research, find out what these are and their relative abundance. Use this information to calculate the relative atomic mass of barium.
Terms/Concepts: Atomic number; Atomic mass; Atomic weight; Isotope; Relative atomic mass
- Gonick, Larry and Craig Criddle. The Cartoon Guide to Chemistry. Collins Reference (2005)
- Moore, John T. Chemistry for Dummies. For Dummies (2002).
- Atomic Number and Mass (http://Galileo.phys.Virginia.EDU/education/outreach/8thgradesol/AtomicNumMass.htm)
- What are Atomic Number and Atomic Weight? (http://www.hss.energy.gov/HealthSafety/ohre/roadmap/achre/intro_9_3.html)